We present detailed theoretical studies of the proton transfer in the isolated, mono, dehydrated forms and dimers of formamide and the effect of hydratation or self-assistance on the transition state structures corresponding to proton transfer from the keto form to the enol form, employing MP2 and B3LYP methods at varied basis sets. The barrier heights for both H2O-assisted and self-assisted reactions are significantly lower than that of the bare tautomerization reaction from formamide to formamidic acid, implying the importance of the superior catalytic effect of H2O, (H2O)(2) and important role of NH2CHO for the intramolecular proton transfer. (C) 2003 Elsevier B.V. All rights reserved.